Geforce 7900 GTX
Codename: G71, 90nm technology
24 Pixel Shaders, Eight Vertex Shaders, 24 Texture Units, 16 Raster Processors
256 bit memory bus
650Mhz core, 800 Mhz DDR (1600 Mhz Effective) Memory
The Geforce 7900 GTX is Nvidia's answer to the X1900 XTX. Both of these heavyweights have their strengths and show superiority in different games and benchmarks, but neither will disappoint. The 7900 GTX will have the advantage in texturing speed, and the X1900 XTX will have the advantage in shader speed.
Radeon X1900 XTX
Codename: R580, 90 nm technology
48 Pixel shaders, Eight Vertex shaders, 16 Texture units, 16 Raster operations processors
256 bit external memory bus (512 bit internal ring bus)
650 MHz core, 775 MHz DDR (1550 MHz effective) Memory
One of the fastest gaming cards on the planet today, the Radeon X1900 XTX boasts slightly higher clock speeds than its XT brother. The Radeon differentiates itself from its 7900 GTX competitor slightly with the ability to use OpenEXR High-Dynamic-Range lighting (HDR) and antialiasing at the same time.
The worst thing we can say about the 7900 GTX and X1900 XTX it is that they're probably not worth the $100 premium over the excellent X1900 XT 256 MB card.
Best PCIe Card For ~$500
Geforce 7950 GX2
Codename: G71, 90 nm technology
2x24 Pixel shaders, 2x8 Vertex shaders, 2x24 Texture units, 2x16 Raster operations processors
256 bit memory bus
500 MHz core, 600 MHz DDR (1200 MHz Effective) Memory
If you have money to burn, the money-is-no-object price segment will offer diminishing returns for the high price tag. But if you have the money and need to play at the highest resolutions available with high levels of AA, then SLI/Crossfire setups are the way to go.
Probably the most reasonably priced and easy to use multi-GPU solution is the Geforce 7950 Gx2, which is actually a single card with two 7900 GPUs built in; it performs faster than any other single card available at ultra-high resolutions.
Keep in mind though that if you play at 1280x1024 resolution with 4xAA, you would be wasting money on the GX2, as cheaper single cards like the X1900 XTX and 7900 GTX can outperform the card at those settings. But if you play at 1600x1200 with 6xAA, the 7950 GX2 can't be beat by any single card out there.